The invention relates generally to a borescope system for use in inspecting drilled shafts, also referred to as bores or boreholes. In particular, the invention relates to a portable visual inspection system for inspecting relatively large drilled construction shafts and the like that provides improved efficiency in terms of maneuverability, information gathering, data recording, data analyzing, and data quantifying.
Drilled construction shafts that are subsequently filled with concrete or similar materials provide support for many large building projects. For this reason, field engineers and inspectors involved in preparing such shafts are particularly concerned with ensuring that the load transfers in side resistance and in end bearing are consistent with any assumptions made during the design phase. Normally, project design methods assume that drilled shafts are constructed under competent supervision and with ample quality control and assume that the finished foundation will be durable and have structural integrity. However, such assumptions are not always warranted. Unless project specifications and procedures are closely followed in the field, for example, the final shaft may have defects that can influence its structural and bearing capacity when filled. Therefore, the inspection of drilled shafts and the record keeping associated with shaft construction are important and require careful attention.
Defects of a finished support structure and the conditions under which such defects occur may involve a number of causes. For example, defects typically result from one or more of the following: 1) over stressing the soil beneath the shaft base due to insufficient bearing (contact) area or because of unconsolidated materials located at the shaft base; 2) excessive mixing from mineral slurry, which can affect the development of concrete strength and/or formation of voids and cavities within the set concrete; and 3) structural discontinuities and/or deviations from the true vertical line causing local, undesirable stress concentrations. In general, these and other defects can result in insufficient load transfer reducing the bearing capacity of the final structure and/or causing excessive settling during service.
To develop the required end bearing capacity, the drilled shaft should be inspected so that undesirable debris may be removed before concrete placement. Shaft failures have been attributed to insufficient borehole cleaning, and cleaning the base of boreholes often requires special tools. Although the operation sounds simple, a typical cleaning process involves several steps including visually inspecting the borehole, sounding the base of the shaft by a weight attached to a chain, and obtaining samples of the side walls and the base. Based on the results of the visual, sounding, and sampling inspections, a trained inspector decides whether the borehole must be cleaned or otherwise altered before concrete placement. The inspector usually bases his or her decision on the condition of the borehole and the amount of sedimentary deposits at the base. If the inspector decides that cleaning is warranted, several methods may be used, including air lifting, using a clean-out-bucket, or removing debris and unwanted material with a submerged pump. The cleaning requirements can be quite strict. For example, the Florida Department of Transportation requires that at least 50 percent of the base of each shaft have less than 0.5 inches (13 mm) of sediment at the time of concrete placement, and that the maximum depth of sedimentary deposits or any other debris at any place on the base of the shaft not exceed 1.5 inches (40 mm).
As may be expected, verifying the conditions existing at the shaft base is often a difficult task. Lowering a human inspector into a borehole, especially one that has been stabilized with slurry, can be very dangerous or even impossible. Thus, to facilitate the inspection process and to avoid sending human inspectors into large construction boreholes, highway agency guidelines often recommend the use of a shaft inspection device. The Florida Department of Transportation, for example, recommends the use of its Shaft Inspection Device (SID), developed in the early 1980s by Schmertmann and Crapps, Inc. The SID comprises a television camera sealed inside a water-tight jacket and is used for inspecting both dry and wet excavations. The concept of the SID was derived from an Australian drilled shaft inspection device originally developed by Dr. Jim Holden of the Country Roads Board.
Since its inception, the SID has been used with only modest success. The SID weighs approximately 10,000 pounds, is quite large, and is relatively expensive. Although the idea of utilizing an optical device to inspect drilled shafts has been favored by engineers and contractors, the operation of present devices like the SID is cumbersome, time consuming, expensive, and often produces disappointing results, especially on drilled shaft projects in waterways. In addition to the high cost of the device itself, the SID's lack of mobility and versatility, particularly in waterways projects, results in higher operating costs.
For these reasons, a portable visual inspection system for drilled shaft inspection with improved efficiency in terms of portability, information gathering, data recording, and quantifying the obtained measurements is desired. Such a system benefits from advancements in many technologies, including imaging, fiber optics, and computers and signal processing, as well as from the development of various types of miniature video scopes and borescope devices.